Integration of photonics optical gyroscopes with micro-electro-mechanical sensors

ABSTRACT

Aspects of the present disclosure are directed to monolithically integrating an optical gyroscope fabricated on a planar silicon platform as a photonic integrated circuit with a MEMS accelerometer on the same die. The accelerometer can be controlled by electronic circuitry that controls the optical gyroscope. Gaps may be introduced between adjacent waveguide turns to reduce cross-talk and improve sensitivity and packing density of the optical gyroscope.

RELATED APPLICATIONS

This application is related to and claims the benefit of U.S.Provisional Patent Application No. 63/047,504, filed Jul. 2, 2020,titled “Integration of Photonics Optical Gyroscopes withMicro-Electro-Mechanical Sensors,” the entirety of which is incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure relates to various structures and fabricationmethods for integrating photonics-based optical gyroscopes withmicro-electro-mechanical systems (MEMS)-based sensors on a same chip. Inother words, this disclosure relates to a photonics integrated circuit(PIC) (also referred to as integrated photonics chip in the claims) witha MEMS-based sensor integrated monolithically to the chip.

BACKGROUND

Gyroscopes (sometimes also referred to as “gyros”) are sensors that canmeasure angular velocity. Gyroscopes can be mechanical or optical, andcan vary in precision, performance cost and size. Mechanical gyroscopesbased on Coriolis effect typically have lower cost, but cannot deliver avery high performance, and are susceptible to measurement errors inducedby temperature, vibration and electromagnetic interference (EMI).Optical gyroscopes typically have the highest performance and rely oninterferometric measurements based on the Sagnac effect (a phenomenonencountered in interferometry that is elicited by rotation). Sinceoptical gyroscopes do not have any moving parts, they have advantagesover mechanical gyroscopes as they can withstand effects of shock,vibration and temperature variation much better than the mechanicalgyroscopes with moving parts.

The most common optical gyroscope is the fiber optical gyroscope (FOG).Construction of a FOG typically involves a long loop (the loop mayconstitute a coil comprising several turns or a fiber spool) ofpolarization-maintaining (PM) fiber. Laser light is launched into bothends of the PM fiber traveling in different directions. If the fiberloop/coil is rotating, the optical beams experience different opticalpath lengths with respect to each other. By setting up aninterferometric system, one can measure the small path length differencethat is proportional to the area of the enclosed loop and the angularvelocity of the rotating coil.

FOGs can have very high precision, but at the same time, they are oflarge dimension, are very expensive, and are hard to assemble due to thedevices being built based on discrete optical components that need to bealigned precisely. Often, manual alignment is involved, which is hard toscale up for volume production. The present disclosure provides asolution to this problem, as described further below.

A plurality of gyroscopes and other sensors (such as accelerometers, andin some cases magnetometers) may be packaged together as an InertialMeasurement Unit (IMU) in a moving object to sense various motionparameters along the X, Y, and Z axes. For example, a 6-axis IMU mayhave 3-axis accelerometers and 3-axis gyroscopes packaged together tomeasure an absolute spatial displacement of the moving object.Applications of IMUs include, but are not limited to, military maneuvers(e.g., by fighter jets, submarines), commercial aircraft/dronenavigation, robotics, autonomous vehicle navigation, virtual reality,augmented reality, gaming etc.

Present inventors propose replacing fibers with waveguide basedintegrated photonics components for cost-effective easy integration on asemiconductor platform which is much more promising for volumeproduction of gyroscopes. This application describes various structuresincluding silicon nitride (SiN) waveguide cores fabricated on a siliconplatform, and integration of a MEMS accelerometer onto the same siliconplatform where the gyroscope is, as elaborated below.

SUMMARY

Aspects of the present disclosure are directed to monolithicallyintegrating an optical gyroscope fabricated on a planar silicon platformas a photonic integrated circuit with a MEMS accelerometer on the samedie. The accelerometer can be controlled by electronic circuitry thatcontrols the optical gyroscope. Gaps may be introduced between adjacentwaveguide turns to reduce cross-talk and improve sensitivity and packingdensity of the optical gyroscope.

Specifically, an integrated photonics chip is disclosed, comprising: awaveguide coil comprising a plurality of waveguide turns looping arounda central area enclosed by the waveguide coil, each waveguide turn beingparallel to adjacent waveguide turns, wherein the waveguide coil is usedas a rotational sensing element of an optical gyroscope; and, amicro-electro-mechanical-systems (MEMS)-based motion sensing devicemonolithically integrated in the central area enclosed by the waveguidecoil, wherein the waveguide coil and the MEMS accelerometer arefabricated on a common platform.

The optical gyroscope and the MEMS-based motion sensing device arepackaged together as a modularized integrated inertial measurement unit(IMU). The MEMS-based motion sensing device provides coarse rotationalsensing reading for all axes of motion, and the optical gyroscope canprovide a higher-precision rotational sensing reading for one or moreselected axes of motion.

The MEMS-based motion sensing device can also comprise an accelerometerfor one or more axes of motion. In some embodiments the MEMS device canbe a six-axis gyroscope and accelerometer.

The common platform of the integrated photonics chip can be a siliconphotonics platform, wherein each waveguide turn comprises a waveguidecore sandwiched between an upper cladding and a lower cladding. In oneembodiment, the waveguide core comprises silicon nitride and the uppercladding and lower cladding comprise oxide.

Structural modification can be introduced on either side of eachwaveguide turn to reduce crosstalk between the adjacent waveguide turns,thereby increasing a spatial density of waveguide turns that can befabricated within a predetermined area of the integrated photonics chip.

The predetermined area may depend on an exposure field of a reticle usedto fabricate the waveguide coil and the MEMS-based motion sensingdevice. Increasing spatial density of waveguide turns increases thecentral area enclosed within the waveguide coil, as well as increases anumber of waveguide turns enclosing the central area, thereby increasingsensitivity of the rotational sensing element.

The structural modification may comprise a gap, wherein the gapcomprises an air gap, a gap filled with metal, or, a gap filled with aninert gas or liquid.

In some embodiments the gap is in the form of a high-aspect-ratiorectangular slit or trench with a longitudinal dimension of the gapbeing substantially higher than a lateral dimension of the gap, suchthat the gap extends substantially above and below the waveguide corealong the longitudinal direction.

Also disclosed is a method for monolithically fabricating an integratedphotonics chip comprising a waveguide coil and a MEMS-based motionsensing device on a common platform, the method comprising: designatinga central area on the common platform to fabricate the MEMS-based motionsensing device, wherein the central area is to be enclosed by thewaveguide coil that comprises a plurality of waveguide turns loopingaround the central area, each waveguide turn being parallel to adjacentwaveguide turns, wherein the waveguide coil is used as a rotationalsensing element of an optical gyroscope; fabricating the waveguide coilon the common platform; protecting the fabricated waveguide coil bydepositing an etch-stop layer above the waveguide coil; and, fabricatingthe MEMS-based motion sensing device in the designated central area.

Fabricating the MEMS-based motion sensing device further comprises:depositing and patterning electrodes on top of the etch-stop layer inthe designated central area; depositing and patterning a firstsacrificial layer on top of the etch-stop layer and the patternedelectrodes; depositing and patterning a structural layer on top of thepatterned sacrificial layer and the patterned electrodes; depositing andpatterning a second sacrificial layer on top of the patterned structurallayer; patterning the structural layer to create columns as parts of theMEMS-based motion sensing device, wherein the second sacrificial layeralso gets patterned on top of the columns; and, removing the first andthe second sacrificial layers, thereby creating a suspending structurethat acts as a motion sensing element of the MEMS-based device.

The method may further comprise forming gaps on either side of eachwaveguide turn to reduce crosstalk between the adjacent waveguide turns,thereby increasing a spatial density of waveguide turns that can befabricated within a predetermined area of the integrated photonics chip.

The integrated photonics chip can have multiple layers or planes, andportions of the photonics components can be distributed among themultiple planes. This way the total footprint of the photonics chip canremain small, but more functionalities can be packed into the photonicschip, as well as more length of the waveguide coil can be introducedwithout increasing the device footprint.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousimplementations of the disclosure.

FIG. 1 is a schematic top view of an optical gyroscope coil (also calleda waveguide coil) with multiple turns and a MEMS accelerometer on thesame die, according to an embodiment of the present disclosure.

FIGS. 2A-2F show the process flow to monolithically integrate the MEMSaccelerometer into the optical gyroscope chip with silicon nitridewaveguides, according to an embodiment of the present disclosure.

Specifically, FIG. 2A shows the cross-sectional view at the startingpoint of a MEMS-SiPhOG integration process flow.

FIG. 2B shows an additional layer of SiN formed on top of the oxidelayer acting as the lower cladding.

FIG. 2C shows a sacrificial layer deposited and patterned on top of theetch stop layer and the patterned electrodes.

FIG. 2D shows a structural layer deposited and patterned on top of thepatterned sacrificial layer and electrodes.

FIG. 2E shows a second sacrificial layer deposited and patterned on topof the structural layer.

FIG. 2F shows patterning of the structural layer to create columnswithin the area designated for the MEMS accelerometer.

FIG. 3 is a schematic cross-sectional view showing the fully processedintegrated device containing the MEMS accelerometer and opticalgyroscope, according to an embodiment of the present disclosure.

FIG. 4 is a schematic cross-sectional view showing an alternativeconfiguration of the gyroscope waveguide chip (cross-sectional view),which can be the basis for monolithically integrating the MEMSaccelerometer, according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

Aspects of the present disclosure are directed to monolithic integrationof compact ultra-low loss integrated photonics-based waveguides withmicro-electro-mechanical system (MEMS)-based sensing devices. Thesewaveguides can be used as optical elements on a planar photonicintegrated circuit (PIC), for example, in photonics integrated opticalgyroscopes. As discussed in the background section, the key tofiber-based optical gyroscopes' high performance is the long length ofhigh quality, low loss, optical fiber that is used to measure the Sagnaceffect. The present inventors recognize that with the advent ofintegrated silicon photonics suitable for wafer scale processing, thereis an opportunity to replace FOGs with smaller integrated photonic chipsolutions without sacrificing performance. Photonics based optical gyroshave reduced size, weight, power and cost, but in addition can be massproduced in high volume, are immune to vibration and electromagneticinterference and have the potential to offer performances equivalent toFOGs. When integrated optical gyroscope is fabricated on a siliconplatform, it is abbreviated as SiPhOG® (Silicon Photonics OpticalGyroscope). This disclosure moves one step more towards bringing sensingelements together by monolithically integrating a MEMS accelerometerinto the photonics gyroscope chip.

One key element of this integrated photonic solution is to produce verylow loss waveguide core made of silicon nitride (Si₃N₄) surrounded byoxide or fused silica claddings. The whole waveguide structure(including core and cladding) is sometimes referred to as SiN waveguidefor simplicity. The propagation loss in the SiN waveguides can be wellbelow 0.1 db/meter. This is a vast improvement over the currentstate-of-the-art SiN process with propagation loss in the range of 0.1db/centimeter.

FIG. 1 shows a SiPhOG®-MEMS combined system 100 fabricated on agyroscope waveguide die 10. Light is launched at a first end 14 of agyroscope waveguide coil 20 with several turns. Here only four turns areshown for clarity, though in a real device, many more turns (forexample, several hundreds of turns) can be used, based on the requiredsensitivity of the gyroscope. After propagating in the waveguide coil,light comes out from a second end 16. Note that since light can belaunched from either end 14 or 16, each of the ends can act as an“input” end or an “output end”. For simplicity, we refer to first end 14as “input end” and second end 16 as “output end”, and refer to theportion 18 of the waveguide closer to the second end 16 as “outputwaveguide” 18. In some embodiments, light can be launched at both ends14 and 16 to obtain phase difference signal from counter-propagatinglight beams. Waveguide coil design takes into account phase interferencebetween counter-propagating beams and/or cross-coupling between adjacentwaveguides, such as between 110 and 112. An accelerometer device 30 isintegrated on the gyroscope waveguide die 10. In one embodiment, anaccelerometer die 12 containing the MEMS accelerometer device 30 can behybridly integrated onto the gyroscope waveguide die 10. For example, apredecessor of this present disclosure is titled, “INTEGRATED PHOTONICSOPTICAL GYROSCOPES OPTIMIZED FOR AUTONOMOUS TERRESTRIAL AND AERIALVEHICLES,” was filed as provisional application 62/923,234 on Oct. 18,2019, where hybrid integration of MEMS sensor with a photonic gyroscopewas described at a module level. That application was converted intonon-provisional application 17/071,697 on Oct. 15, 2020, which has beenpublished as U.S. 2021/0116246. All of these applications areincorporated herein by reference. The present disclosure focuses onin-plane integration of the MEMS accelerometer device 30 onto thegyroscope waveguide die 10. In other words, a self-contained inertialmeasurement unit (IMU) containing photonics optical gyroscope and MEMSaccelerometer can be fabricated as a single device on a single die,which is sometimes referred to as SiPhOG-X.

FIG. 2A shows the cross-sectional view at the starting point of aMEMS-SiPhOG integration process flow. Specifically, FIG. 2A shows asubstrate 102, which may be a silicon substrate. The substrate 102 mayhave a thickness of a standard wafer, e.g., the thickness can be 725 um.Note that the thickness of different material layers are not drawn toscale. However, in order to convey the idea that the substrate 102 ismuch thicker than the rest of the material layers shown in the FIGS.2A-2F, the discontinuity 101 is introduced in the middle of the layer102 just for visualization. The layers 106 and 116 can have a thickness‘h1’ in the range of 15 um on both sides of the substrate 102. Layer 106acts as a lower cladding for the waveguide cores 110 and 112. Note thatadjacent waveguide cores 110, 112 correspond to each turn of thewaveguide coil 20 shown in FIG. 1. The pitch p1 between the cores may bein the range of 20 um, which can be reduced significantly by introducingstructural modification in between the adjacent waveguide cores, asdescribed later with respect to FIG. 4. Waveguide cores 110 and 112 canhave a thickness ‘h’ and width ‘w’. Non-limiting exemplary dimensionsfor ‘h’ can be 60-100 nm, and ‘w’ can be 2-3 um. Waveguide cores 110 and112 are made of silicon nitride (SiN). An upper cladding 104 is formedon top of waveguide cores 110 and 112. The thickness ‘h2’ of the uppercladding layer 104 can be in the range of 2-3 um. The thickness may bedecreased to about 1 um if necessary for the subsequent process flow tocreate the MEMS accelerometer. Note that when layers 106, 110 (and 112)and 104 are formed on one side of substrate 102, corresponding layers116, 118 and 114 are also formed on the other side of the substrate 102,even though those layers are not used for waveguiding purposes.Alternatively, those layers can create waveguides in a different layer,if necessary. Both upper and lower claddings 104 and 106 are shown to beof the same material 108, e.g. silicon oxide, though in variousembodiments, the lower cladding can be pre-grown oxide and the uppercladding may be deposited oxide, such as TEOS (Tetra Ethyl OrthoSilicate) or other composition. Similarly both layers 114 and 116 havethe same material 120 which is identical to material 108. The dashedoutline 113 shows area earmarked for the turns of the optical gyroscopecoil 20 on the die 10, while the dashed outline 111 shows the areaearmarked for subsequent fabrication of the MEMS accelerometer on thesame die 10.

FIG. 2B shows an additional layer 122 of SiN formed on top of the oxidelayer 108. This SiN layer 122 acts as an etch stop layer for the MEMSaccelerometer fabrication. Electrodes 124 and 126 can be deposited andpatterned on top of the layer 122 within the area 111 designated for theMEMS accelerometer.

FIG. 2C shows a sacrificial layer 128 deposited and patterned on top ofthe etch stop layer 122 and the patterned electrodes 126 and 124.Sacrificial layer 128 may be an oxide layer.

FIG. 2D shows a structural layer 130 deposited and patterned on top ofthe patterned sacrificial layer 128 and electrodes 126 and 124.Structural layer 130 can be made of poly-silicon-germanium (SiGe). Thislayer fills the gaps between the patterned sacrificial layer 128.

FIG. 2E shows a second sacrificial layer 131 deposited and patterned ontop of the structural layer 130. Layer 131 may be the same material aslayer 128, e.g. oxide. Bond pads 132 and 134 are formed and patterned ontop of the second sacrificial layer 131.

FIG. 2F shows patterning of the structural layer 130 to create columns130 a, 130 b, 130 c, 130 d and 130 e within the designated area 111 forthe MEMS accelerometer. The second sacrificial layer 131 also getspatterned on top of the columns (131 a, 131 b, 131 c).

FIG. 3 shows the MEMS device 30 after the sacrificial layers 128 and 131are removed, thereby creating the suspending structure 130 c in themiddle between the surrounding frame denoted by columns 130 b and 130 d.This freely suspending structure 130 c is essential for the operation ofthe MEMS accelerometer.

Note that each waveguide core 110, 112 corresponds to each turn of thewaveguide coil 20 shown in FIG. 1. To maintain single mode and to avoidcoupling between adjacent waveguides, a minimum pitch p1 needs to bemaintained between the adjacent waveguide cores. A non-limiting examplevalue of p1 can be 14-16 um, or even 20 um. This pitch limits the totalserial length of the waveguide on a die 10 (see FIG. 1), and the maximumarea enclosed by the waveguide coil 20 is also limited.

FIG. 4 shows one approach to mitigating the crosstalk between adjacentwaveguides while making the pitch shorter, thereby being able to moredensely pack the turns of the waveguide coil 20. The embodiment in FIG.4 automatically leads to longer total length of the waveguide and alarger enclosed area, that translate to higher sensitivity of theoptical gyroscope. Introducing air gaps 450 on both sides of thewaveguide cores confines optical modes largely within one turn of thewaveguide and prevents leakage of optical signal to the adjacent turn ofthe waveguide. In other words, the air gaps 450 act as physicalisolations between adjacent waveguide cores 410, 412, 414, and 416. Asthe pitch p2 between two adjacent waveguide cores reduces, automaticallymore turns can be accommodated within the same reticle field, increasingboth the total length and the enclosed area in an individual layer orplane. The pitch p2 can very well be less than 10 um with the air gaps,i.e. much lower than the pitch p1 shown in FIG. 2A. Note that instead ofair, the gaps may be filled with other non-reactive fluid, such as aninert gas. Also, instead of air gaps, sub-wavelength grating-likestructures or metal barriers can be used in between adjacent waveguidesto reduce pitch without increasing cross talk. Note that a MEMSaccelerometer can be fabricated on the modified SiN waveguide chip (e.g.with air gaps) much in the same way as described with respect to FIGS.2A-2F.

Note that one option can be distributing the total length of a SiNwaveguide coil with multiple turns (and/or a ring with a single turn)into different vertically separated layers (e.g., two or more layers)that would lead to improved gyro sensitivity without increasing the formfactor. Details of a stacked multi-layer gyro configuration are coveredin provisional application 62/858,588 filed on Jun. 7, 2019, titled,“Integrated Silicon Photonics Optical Gyroscope on Fused SilicaPlatform.” A follow-up provisional application 62/896,365 filed on Sep.5, 2019, titled “Single-layer and Multi-layer Structures for IntegratedSilicon Photonics Optical Gyroscopes” describes additional embodiments.A third provisional application 62/986,379, titled, “Process Flow forFabricating Integrated Photonics Optical Gyroscopes,” was filed on Mar.6, 2020. These three applications were combined into a non-provisionalapplication 16/894,120 filed on Jun. 5, 2020, titled “Single-layer andMulti-layer Structures for Integrated Silicon Photonics OpticalGyroscopes” which eventually issued as U.S. Pat. No. 10,969,548 on Apr.6, 2021. These applications are incorporated herein by reference. Inaddition, system-level integration of a silicon photonics basedfront-end chip and a SiN waveguide chip have been covered in provisionalapplications 62/872,640 filed Jul. 10, 2019, titled “System Architecturefor Silicon Photonics Optical Gyroscopes”, and 62/904,443 filed Sep. 23,2019, titled, “System Architecture for Silicon Photonics OpticalGyroscopes with Mode-Selective Waveguides.” These two applications werecombined into a non-provisional application 16/659,424 filed on Oct. 21,2019, titled “System Architecture for Silicon Photonics OpticalGyroscopes” which eventually issued as U.S. Pat. No. 10,731,988 on Aug.4, 2020. These applications are also incorporated herein by reference.

However, in the above applications, the need to manufacture a two-layerdevice arose partly because in a single plane, the adjacent waveguidesneed to be spaced apart at a pitch that prevents unwantedcross-coupling. Therefore, to keep the footprint of the device more orless same, the total length of the waveguide spiral was distributedbetween more than one planes. This present disclosure provides solutionswhere adjacent waveguides can be packed more tightly in a single plane,i.e. the pitch between adjacent waveguides is reduced in an individualplane. Note that the terms “layer” and “plane” have been usedinterchangeably when describing distributing the waveguide coil intomultiple planes. Densely packing waveguides on a single plane mayobviate the need to fabricate a multi-layer device altogether, or atleast can reduce the number of layers necessary to get a suitable totallength of waveguide coil that is directly related to the sensitivity ofthe optical gyroscope.

In summary, incorporating MEMS sensors in the same chip as the photonicsoptical gyroscope utilizes both the Coriolis force and the Sagnac effectto produce precision inertial sensing, including rotation andacceleration sensing. Even low-precision mechanical gyroscopes can beintegrated on the same die for axes that do not need precision opticalreadout produced by the Sagnac effect gyroscopes. Monolithicallyintegrating SiPhOG and MEMS sensors makes it earlier to bring all theelectronic control circuitry for the various sensors on the same chip.

It is noted that some sensing applications may need high-precisionoptical gyroscope for just one axis to supplement or replacelow-precision measurement by a low-cost mechanical gyroscope (such as aMEMS-based gyroscope), while the other two axes may continue to uselow-precision measurement from low-cost mechanical gyroscopes. One suchexample is gyroscopes in safety sensors relied upon by automatic driverassistance systems (ADAS) for current and future generations ofautonomous vehicles, especially for Level 2.5/Level 3 (L2.5/L3) markets.In ADAS, high-precision angular measurement may be desired only forZ-axis (the yaw axis) for determining heading, because the vehicle stayson the X-Y plane of a rigid road. The angular measurement for the X andY axis (pitch and roll axes) may not be safety-critical in thisscenario. The present inventors recognize that by bringing down the costof high precision optical gyroscopes at least for one axis translates tooverall cost of reduction of the IMU that would facilitate mass marketpenetration. Additionally, as needed, the mechanical gyroscopes in theother two axes may also be replaced or supplemented by opticalgyroscopes with proper design of system level integration in all 3 axes(pitch, roll and yaw axes), for example in unmanned aerial vehicles(e.g., drones), construction, farming, industrial, marine vehicles,L4/L5 markets and certain military applications.

In the foregoing specification, implementations of the disclosure havebeen described with reference to specific example implementationsthereof. It will be evident that various modifications may be madethereto without departing from the broader spirit and scope ofimplementations of the disclosure as set forth in the following claims.The specification and drawings are, accordingly, to be regarded in anillustrative sense rather than a restrictive sense. Additionally, thedirectional terms, e.g., “top”, “bottom” etc. do not restrict the scopeof the disclosure to any fixed orientation, but encompasses variouspermutations and combinations of orientations.

1. An integrated photonics chip comprising: a waveguide coil comprisinga plurality of waveguide turns looping around a central area enclosed bythe waveguide coil, each waveguide turn being parallel to adjacentwaveguide turns, wherein the waveguide coil is used as a rotationalsensing element of an optical gyroscope; and amicro-electro-mechanical-systems (MEMS)-based motion sensing devicemonolithically integrated in the central area enclosed by the waveguidecoil, wherein the waveguide coil and the MEMS accelerometer arefabricated on a common platform.
 2. The integrated photonics chip ofclaim 1, wherein the optical gyroscope and the MEMS-based motion sensingdevice are packaged together as a modularized integrated inertialmeasurement unit (IMU).
 3. The integrated photonics chip of claim 2,wherein the MEMS-based motion sensing device provides coarse rotationalsensing reading for all axes of motion, and the optical gyroscopeprovides a higher-precision rotational sensing reading for one or moreselected axes of motion.
 4. The integrated photonics chip of claim 2,wherein the MEMS-based motion sensing device comprises an accelerometerfor one or more axes of motion.
 5. The integrated photonics chip ofclaim 1, wherein the common platform is a silicon photonics platform,wherein each waveguide turn comprises a waveguide core sandwichedbetween an upper cladding and a lower cladding.
 6. The integratedphotonics chip of claim 5, wherein the waveguide core comprises siliconnitride and the upper cladding and lower cladding comprise oxide.
 7. Theintegrated photonics chip of claim 5, further comprising: a structuralmodification introduced on either side of each waveguide turn to reducecrosstalk between the adjacent waveguide turns, thereby increasing aspatial density of waveguide turns that can be fabricated within apredetermined area of the integrated photonics chip.
 8. The integratedphotonics chip of claim 7, wherein the predetermined area depends on anexposure field of a reticle used to fabricate the waveguide coil and theMEMS-based motion sensing device.
 9. The integrated photonics chip ofclaim 7, wherein increasing spatial density of waveguide turns increasesthe central area enclosed within the waveguide coil, as well asincreases a number of waveguide turns enclosing the central area,thereby increasing sensitivity of the rotational sensing element. 10.The integrated photonics chip of claim 7, wherein the structuralmodification comprises a gap.
 11. The integrated photonics chip of claim10, wherein the gap comprises one of: an air gap, a gap filled withmetal, or, a gap filled with an inert gas or liquid.
 12. The integratedphotonics chip of claim 10, wherein the gap is in the form of ahigh-aspect-ratio rectangular slit or trench with a longitudinaldimension of the gap being substantially higher than a lateral dimensionof the gap, such that the gap extends substantially above and below thewaveguide core along the longitudinal direction.
 13. The integratedphotonics chip of claim 1, wherein a first portion of the waveguide coilresides on a first plane and a second portion of the waveguide coilresides on a second plane, wherein the first plane and the second planeare vertically stacked above one another.
 14. The integrated photonicschip of claim 13, wherein light couples evanescently between the firstportion of the waveguide coil on the first plane and the second portionof the waveguide coil on the second plane.
 15. A method formonolithically fabricating an integrated photonics chip comprising awaveguide coil and a MEMS-based motion sensing device on a commonplatform, the method comprising: designating a central area on thecommon platform to fabricate the MEMS-based motion sensing device,wherein the central area is to be enclosed by the waveguide coil thatcomprises a plurality of waveguide turns looping around the centralarea, each waveguide turn being parallel to adjacent waveguide turns,wherein the waveguide coil is used as a rotational sensing element of anoptical gyroscope; fabricating the waveguide coil on the commonplatform; protecting the fabricated waveguide coil by depositing anetch-stop layer above the waveguide coil; and fabricating the MEMS-basedmotion sensing device in the designated central area.
 16. The method ofclaim 15, wherein fabricating the MEMS-based motion sensing devicefurther comprises: depositing and patterning electrodes on top of theetch-stop layer in the designated central area.
 17. The method of claim16, further comprising: depositing and patterning a first sacrificiallayer on top of the etch-stop layer and the patterned electrodes;depositing and patterning a structural layer on top of the patternedsacrificial layer and the patterned electrodes; and depositing andpatterning a second sacrificial layer on top of the patterned structurallayer.
 18. The method of claim 17, further comprising: patterning thestructural layer to create columns as parts of the MEMS-based motionsensing device, wherein the second sacrificial layer also gets patternedon top of the columns; and removing the first and the second sacrificiallayers, thereby creating a suspending structure that acts as a motionsensing element of the MEMS-based device.
 19. The method of claim 15,further comprising: forming gaps on either side of each waveguide turnto reduce crosstalk between the adjacent waveguide turns, therebyincreasing a spatial density of waveguide turns that can be fabricatedwithin a predetermined area of the integrated photonics chip.
 20. Themethod of claim 15, wherein fabricating the waveguide coil furthercomprises: fabricating a first portion of the waveguide coil on a firstplane; and fabricating a second portion of the waveguide coil on asecond plane, wherein light couples evanescently between the firstportion of the waveguide coil on the first plane and the second portionof the waveguide coil on the second plane.